Plants have two basic growth modes during their life cycles—vegetative growth and flower and seed growth. Above ground vegetative growth of the plant develops from the apical meristem. This vegetative meristem gives rise to all of the leaves that are found on the plant. The plant will maintain its vegetative growth pattern until the apical meristem undergoes a change. This change actually alters the identity of the meristem from a vegetative to an inflorescence meristem. The inflorescence meristem produces small leaves before it next produces floral meristems. It is the floral meristem from which the flower develops.
From a genetic perspective, two phenotypic changes that control vegetative and floral growth are programmed in the plant. The first genetic change involves the switch from the vegetative to the floral state. If this genetic change is not functioning properly, then flowering will not occur. The second genetic event follows the commitment of the plant to form flowers. The observation that the organs of the plant develop in a sequential manner suggests that a genetic mechanism exists in which a series of genes are sequentially turned on and off.
Flowering time is an important agronomic trait in cultivated plant species as it determines in large measure the growing region of adaptation. Most angiosperm species are induced to flower in response to environmental stimuli such as day length and temperature, and internal cues, such as age. Genetic analysis revealed that there are several types of mutants that alter flowering time.
Studies of two distantly related dicotyledons, Arabidopsis thaliana and Antirrhinum majus, has led to the identification of three classes of homeotic genes, acting alone or in combination to determine floral organ identity (Bowman, et al., Development, 112:1, 1991; Carpenter and Coen, Genes Devl., 4:1483, 1990; Schwarz-Sommer, et al., Science, 250:931, 1990). Several of these genes are transcription factors whose conserved DNA-binding domain has been designated the MADS box (Schwarz-Sommer, et al., supra).
Earlier acting genes that control the identity of flower meristems have also been characterized. Flower meristems are derived from inflorescence meristems in both Arabidopsis and Antirrhinum. Two factors that control the development of meristematic cells into flowers are known. In Arabidopsis, the factors are the products of the LEAFY gene (Weigel, et al., Cell 69:843, 1992) and the APETALA1 gene (Mandel, et al., Nature 360:273, 1992). When either of these genes is inactivated by mutation, structures combining the properties of flowers and inflorescence develop (Weigel, et al., supra; Irish and Sussex, Plant Cell, 2:741, 1990). In Antirrhinum, the homologue of the Arabidopsis LEAFY gene is FLORICAULA (Coen, et al., Cell, 63:1311, 1990) and that of the APETALA1 gene is SQUAMOSA (Huijser, et al., EMBO J., 11:1239, 1992). The latter pair contains MADS box domains.
Genetic studies in Arabidopsis thaliana have identified five genes (APETALA1 (AP1), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG)) that are involved in the specification of floral organ identity. Mutations in these genes result in homeotic transformation of one organ type into another, much like the homeotic selector genes in animal development. These five genes act in spatially localized domains in a flower and in different combinations to specify the development of the sepals, petals, stamens, and carpels. All five genes encode proteins which appear to function as transcription factors. Four of these proteins are members of the MADS domain family of dimeric transcription factors. MADS domain proteins are found in many organisms including yeast, mammals, insects, and plants. The fifth protein, AP2, is a member of another class of DNA binding proteins which may be unique to plants
APETALA2 (AP2) plays an important role in the control of Arabidopsis flower and seed development and encodes a putative transcription factor that is distinguished by a novel DNA binding motif referred to as the AP2 domain. The AP2 domain containing or RAP2 (related to AP2) family of proteins is encoded by a minimum of 12 genes in Arabidopsis. The RAP2 genes encode two classes of proteins, AP2-like and EREBP-like, that are defined by the number of AP2 domains in each polypeptide as well as by two sequence motifs referred to as the YRG and RAYD elements that are located within each AP2 domain. RAP2 genes are differentially expressed in flower, leaf, inflorescence stem, and root. Moreover, the expression of at least three RAP2 genes in vegetative tissues are controlled by AP2. Thus, unlike other floral homeotic genes, AP2 is active during both reproductive and vegetative development.
Maize is a monocotyledonous plant species and belongs to the grass family. It is unusual for a flowering plant as it has unisexual inflorescences The male inflorescence (tassel) develops in a terminal position, whereas the female inflorescences (ears) grow in the axil of vegetative leaves. The inflorescences, as typical for grasses, are composed of spikelets. In the case of maize each spikelet contains two florets (the grass flower) enclosed by a pair of bracts (inner and outer glume). A number of genes have been identified which modify flowering time in maize including Id1 and DLF.
There is increasing incentive by those working in the field of plant biotechnology to successfully genetically engineer plants, including the major crop varieties. One genetic modification that would be economically desirable would be to accelerate the flowering time of a plant. Induction of flowering is often the limiting factor for growing crop plants. One of the most important factors controlling induction of flowering is day length, which varies seasonally as well as geographically. There is a need to develop a method for controlling and inducing flowering in plants, regardless of the locale or the environmental conditions, thereby allowing production of crops, at any given time. Since most crop products (e.g. seeds, grains, fruits), are derived from flowers, such a method for controlling flowering would be economically invaluable.
It is an object of the present invention to provide methods and compositions for affecting flowering time in plants.
It is yet another object of the invention to provide novel nucleotide sequences isolated from maize which encode proteins which affect flowering time in plants.
It is yet another object of the invention to provide maize RAP2.7 genes which affect flowering time and internode length in maize.
It is yet another object of the invention to provide DNA regulatory factors which enhance/inhibit the ability of RAP2.7 to regulate flowering time.
It is yet another object of the invention to provide methods and compositions including nucleotide constructs, vectors, transgenic cells and plants with altered flowering characteristics as described herein.
It is yet another object of the invention to provide markers for identification of mutant plants which may have altered flowering time by the presence of marker VGT1 sequences.